The invention relates to alpha-numeric character display by electrical means (e.g. on the screen of a television receiver or using an electrically-operated printer device), and more especially to a character generator arrangement as used in a character display system for producing character generating data for causing the display of characters by the system.
It is known to provide in such a character generator arrangement a character store (e.g. a "read-only" memory device) in which is stored character information pertaining to characters which conform to a character format contained within a co-ordinate matrix of discrete character elements arranged in rows and columns. In response to character input data representing alpha-numeric characters selected for display, the character store produces character information which can be used to effect the display.
Such a character generator arrangement can be of a kind which comprises, in addition to the character store, input addressing means for addressing the character store with character input data for a character selected for display, row selection means for producing row addresses each of which is for selecting for a read-out operation from the character store the character information pertaining to a particular character element row of the character matrix, and data output means for producing in said read-out operation the character generating data for the character element(s) in the selected character element row of the selected character.
It is known for a character generator arrangement of the above kind to further comprise logic control means for so controlling the input addressing means and the row selection means as to cause, for a plurality of characters to be displayed as a row of characters, the production of the character generating data for the character elements in a first character element row of each of said plurality of characters in turn, then the production of the character generating data for the character elements in a second character element row thereof in turn, and so on in respect of subsequent character element rows, so that said plurality of characters are built-up together element row-by-element row for display.
For television display (i.e. using raster scanning of a television picture tube), the character generating data is used to modulate the picture tube beam current to selectively brighten-up elemental areas on the television screen to form the displayed characters. The horizontal width of a character displayed in the above manner will be determined by the length of the elemental areas. This horizontal width can be varied without altering the standard line and frame scans for the television picture tube. The vertical height of a display character will be determined by the line spacing of the television screen, so that the character height cannot be varied, using only a single television line for each character element row, unless the television line spacing is varied.
However, it is known to increase the character height (to a conventional display size) by arranging the logic control means to cause the character generator arrangement to produce character generating data appropriate for displaying each row of character elements twice on successive television lines. For non-interlaced raster scanning this involves repeating each row of character elements once in the next successive line scan. For interlaced raster scanning, in which two interlaced fields are used to make up a frame scan, each row of characters is produced in each field, resulting in the interlaced display of characters.
The interlaced raster scanning technique is used inter alia in television receiver apparatus in or for use in a television system of a character in which coded data pulses pertaining to alpha-numeric text or other message information are transmitted in a video signal in at least one television line of television frames where no picture signals pertaining to normal picture information are present. At present, it is envisaged that such transmission would be only in field-blanking intervals, but the possibility does exist for so-called "full-field" transmission in which all the television lines of each field are used for transmitting coded data pulses: in other words an entire television channel is dedicated to the data transmission.
A television system of the above character in which the data transmission is restricted to the field-blanking intervals is described in United Kingdom Patent Specification No. 1,370,535. Television receiver apparatus for the system is known from Mullard Technical Information Article 34, dated Sept. 1976 and Mullard Technical Information Article 54, dated August 1977.
A conventional television receiver for the system includes or has associated with it such television receiver apparatus which comprises a data acquisition circuit to extract the coded data pulses from a received video signal. The extracted coded data pulses are stored in a storage device of the apparatus and after a plurality of frame periods an entity of related message information, for example a page of text, has been received and stored. The television receiver apparatus also includes a character generator arrangement of the kind set forth above for converting the stored message information, as applied to this arrangement as the character input data, into the character generating data which is produced in the form of a video signal to cause the display of the message information at the television receiver.
Examples of a television system of the character referred to are the CEEFAX and ORACLE systems used by the BBC and IBA, respectively, for transmitting Teletext data within the broadcast standards of the 625-line domestic television system as employed in the United Kingdom. The standards laid down for these CEEFAX and ORACLE systems are given in "Broadcast Teletext Specification", Sept. 1976, published jointly by the British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturer's Association. These standards specify inter alia that in a page of text there shall be up to 24 rows of characters with up to 40 characters per row. This standard can be achieved in a 625-line television frame by using a character format of 7.times.5 character elements contained within a 10.times.6 character element matrix to cater for the spacing required between adjacent characters in a row and between adjacent rows of characters. Using this character element matrix for the interlaced display of characters therefore requires 24.times.10=240 television lines in each field, so that 240.times.2=480 television lines of a 625-line frame are required to display a page of characters. This number of television lines is well within the maximum number allowed for a visible picture frame, taking into account the use of 50 television lines for the two field blanking intervals per frame, and an overscanning margin of, say 3% in the scanning for the visible picture frame to compensate for small variations in component values or voltages in a television receiver.
Television receiver apparatus which is suitable for Teletext data reception and processing is extremely complex, so that there is an economical advantage in implementing such apparatus in integrated circuit form. However, dedicated integrated circuits which are presently available and are suitable for implementing television receiver apparatus for the 625-line CEEFAX and ORACLE systems, have the limitation that they are unsuitable for implementing television receiver apparatus for Teletext-type data systems which would use another line standard, for instance the 525-line standard which is used widely in the U.S.A.
A particular aspect of such a limitation is that using a character generator arrangement of the kind set forth above which functions to provide the character generating data for 24 rows of character per page of text, it would be possible to display only 20 rows of characters per page of text in a system using the 525-line standard. This is because the 480 television lines which are required for the 24 rows of characters per page of text would not allow a sufficient margin for overscanning, once the television lines which are required for the two field blanking intervals per frame have seen subtracted from the 525-lines per frame which are available. As a result, character rows at the top and bottom of a page, are likely to be outside the visible picture frame. It follows that the entire page format of a Teletext-type data system using a 525-line standard would have to be altered unless the character generator arrangement can be organized to provide character generating data for 24 rows of characters per page of text in a system using a 525-line standard.